1. Field of the Invention
This invention relates to a process for controlling an aeration tank in an apparatus for sewage treatment by activated sludge.
The apparatus for sewage treatment by activated sludge is widely used for removing organic matter in municipal sewage and industrial waste water. The sewage treatment by activated sludge is a process for converting the organic matter in the sewage to sludge by an assimilating action for a group of microorganisms called activated sludge by blowing an oxygen-containing gas, for example, air into an aeration tank, thereby supplying oxygen thereto. The aerated sewage is led to a sedimentation tank from the aeration tank to concentrate and separate the sludge from water, and the concentrated sludge is returned to the aeration tank, or withdrawn to the outside of the apparatus as excess sludge, whereas supernatant water is discharged as treated effluent water.
One of the most important problems in such an apparatus for sewage treatment as described above is control of the aeration tank, that is, control of flow rate of sludge and flow rate of oxygen to the most suitable state for promoting the assimilating action of the microorganisms in the aeration tank.
The present invention concerns a process for controlling an aeration tank by detecting a concentration of nitrous oxide (N.sub.2 O) or carbon dioxide (CO.sub.2) in an exhaust gas from the aeration tank, controlling the activation state of the aeration tank on the basis of the detected concentration, and maintaining the aeration tank in a suitable state for the assimilating action of the microorganisms therein.
2. Description of the Prior Art
In order to efficiently carry out sewage treatment by activated sludge, it is necessary:
(1) to convert the organic matter in the sewage to activated sludge as much as possible, and PA1 (2) to maintain activated sludge with good settling characteristic, and make the activated sludge fully settlable in the sedimentation tank, thereby making the treated water (effluent water) free from the organic matter.
In order to meet the requirement (1), it is preferable to detect a concentration of organic matter in sewage and that in treated water. Generally, a concentration of organic matter is represented and measured by chemical analysis or by a total organic carbon meter (total organic carbon will be hereinafter referred to as TOC). However, the chemical analysis is based on a manual procedure, and requires much time in measurement, and thus it is difficult to utilize the chemical analysis in controlling an aeration tank. The TOC meter determines a concentration of organic matter in sewage, and also requires several ten minutes for the measurement, and thus it is difficult to utilize it in on-line control.
In order to meet the requirement (2), it is necessary to maintain the ratio of organic matter to the unit amount of sludge, that is, a load of organic matter, at an appropriate value, but this ultimately leads to the detection of a concentration of organic matter in sewage.
On the other hand, the art of controlling an aeration tank is disclosed in U.S. Pat. No. 3,684,702 entitled "method and apparatus for determining the biochemical decomposability of sewage", which relates to optimum control of biological decomposition in an aeration tank by measuring a biochemical oxygen demand (BOD) of sewage and controlling return sludge flow rate.
A case of a digestor, though quite different from the subject matter of the present invention as will be briefly mentioned below, by indirectly measuring the state of the tank, and controlling the tank, is disclosed in U.S. Pat. No. 4,062,770 entitled "Method of and apparatus for digesting organic waste and/or sewage sludge", whose subject matter is a process for organic solid waste or sewage sludge (solid matter), that is, a digestor, and is quite different from the activated sludge process as in the present invention. The digestor is based on dissimilation or sludge decomposition, whereas the activated sludge process is based on assimilation or cell synthesis to convert the soluble organic matter in sewage to activated sludge.